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OLED Technology
Submitted By PRIYANSHU SINHA (2K21/B9/05)
Under the guidance of Prof. Pragati Kumar
For the evaluation of MTE Project under the course EE-101.
What is an OLED?
OLED’s are simple solid-state devices (more of an LED) comprised of very thin films of organic
compounds in the electro-luminescent layer. These organic compounds have a special property
of creating light when electricity is applied to it. The organic compounds are designed to be in
between two electrodes. Out of these one of the electrodes should be transparent. The result is a
very bright and crispy display with power consumption lesser than the usual LCD and LED.
History
The first OLED device was developed by Eastman Kodak in 1987.
In 1996, pioneer produces the world’s first commercial PMOLED.
In 2000, many companies like Motorola, LG etc developed various displays.
In 2001, Sony developed world’s largest fullcolor OLED.
In 2002, approximately 3.5 million passive matrix OLED sub-displays were sold, and
over 10 million were sold in 2003.
In 2010 and 2011, many companies announced AMOLED displays.
Many developments had take place in the year 2012.
How does an OLED Works?
OLEDs work in a similar way to conventional diodes and LEDs, but
instead of using layers of n-type and p-type semiconductors, they
use organic molecules to produce their electrons and holes. A simple
OLED is made up of six different layers.
On the top and bottom there are layers of protective!glass!or!plastic.
The top layer is called the!seal!and the bottom layer the!substrate.
In between those layers, there's a!negative terminal!(sometimes
called the cathode) and a!positive terminal!(called the anode).
Finally, in between the anode and cathode are two layers made from
organic molecules called the!emissive layer!(where the light is
produced, which is next to the cathode) and the!conductive
layer!(next to the anode).
OLED’s Materials
Materials are critical factor for both efficiency and lifetime of OLED.
Revolutionary improvement of OLED efficiency can be applied by
introducing new materials. From the first generation of fluorescent
materials until the novel transport and emission layer materials, the
efficiency OLED has been grown more than tenfold. Continuous
development of OLED materials also allowed devices to operate with
a lifetime of hundreds of thousands of hours. Besides, increasing of
layers in OLED also helps for carriers’ injection and charge blocking
effect from reaching opposite side of OLED.
Substrate
The substrate is used to support OLED. The most common materials for
substrate are glass, plastic, and foil which are transparent for the light to emit.
Anode
The anode usually uses highly transparent Indium Tin Oxide (ITO) for light
visibility. This material also a good conductor with high work function in order to
promote the injection of holes into HOMO level of organic layer. Graphene based
anode may be the future conductive anode which exhibit almost same properties
as ITO and used to replace current ITO transparent anode.
Hole Transport Layer (HTL)
P-type materials are typically chosen as HTL. These material are poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), N,N-diphenyl-
N,N-bis(3-methylphenyl)-1,10-biphenyl-4,4-amine (TPD), and N,N-
Bis(naphthalen-1-yl)-N,N-bis(phenyl)benzidine (NPB).
Emissive Layer (EML)
The EML is made up of organic plastic molecules. The colour of light produced is depending on
the type of organic molecule used for the process. Besides, light produced also depends on the
intensity. The more the current applied, the more brighter the light produced due to recombination
of electron-hole pairs and form excitons. Most commonly used material is polyfluorene (PFO)
from fluorescent dye and phosphorescent dye. Poly[2-methoxy-5-(2-ethyl-hexyloxy)phenylene
vinylene] (MEH-PPV) also widely used to produce the light orange-red in colour [14].
Electron Transport Layer (ETL)
N-type materials are typically chosen as ETL. Most common components used are
polyethylenimine ethoxylated (PEIE), 2-(4-Biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD), tris(8-quinolinolato) aluminium (Alq3), and 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP).
Cathode
The cathode is deposited with aluminium (Al) in common, but it still depends on the type of OLED
used. The cathode can be either transparent or solid metal type, such as barium, calcium,
aluminium, gold, and ITO. Conductor with lesser work function will be chosen as cathode in order
to improve electron injection.
How this Sandwich of layers
produce light?
Transparent
substrate
Anode(IT
O)
Conductive layer
Emissive layer
Cathode
LUMO
LUMO
HOMO
HOMO
eˉ
eˉ
h+
h+
h+
Light
During operation, a voltage is applied across the OLED such that the anode is positive
with respect to the cathode. Anodes are picked based upon the quality of their optical
transparency, electrical conductivity, and chemical stability. A current of!electrons!flows
through the device from cathode to anode, as electrons are injected into the LUMO of the
organic layer at the cathode and withdrawn from the HOMO at the anode. This latter
process may also be described as the injection of!electron holes!into the HOMO.
Electrostatic forces bring the electrons and the holes towards each other and they recombine
forming an!exciton, a bound state of the electron and hole. This happens closer to the electron-
transport layer part of the emissive layer, because in organic semiconductors holes are
generally more!mobile!than electrons. The decay of this excited state results in a relaxation of
the energy levels of the electron, accompanied by emission of!radiation!whose!frequency!is in
the!visible region. The frequency of this radiation depends on the!band gap!of the material, in
this case the difference in energy between the HOMO and LUMO.
Indium tin oxide!(ITO) is commonly used as
the anode material. It is transparent to visible
light and has a high!work function!which
promotes injection of holes into the HOMO
level of the organic layer.
We can make an OLED produce coloured light by adding a coloured filter into our plastic
sandwich just beneath the glass or plastic top or bottom layer. If we put thousands of red, green,
and blue OLEDs next to one another and switch them on and off independently, they work like the
pixels in a conventional LCD screen, so we can produce complex, hi-resolution coloured pictures.
Advantages
• Self-luminous: OLEDs are self-luminous and thus do not require backlight, diffusers, or
polarisers.
•Low Power: 2-10 Volts (DC) Low cost and easy fabrication: Roll-to-roll manufacturing
process, such as inkjet printing and screen printing, are possible for polymer OLEDs
•Colour selectivity: There are abundant organic materials to produce a whole spectrum of
visible light.
•Light weight, compact and thin devices: OLEDs are generally very thin.
•Flexibility: OLEDs can be easily fabricated on plastic substrates paving the way for
flexible electronics.
•High brightness and high resolution: OLEDs are very bright at low operating voltage
e.g. white OLEDs can be as bright as 150,000 cd/m2 .
•Wide viewing angle: OLED emission is lambertian and so the viewing angle is as high as
160 degrees.
•Fast response: OLEDs electroluminescence decay time is less than one microsecond.
•Highly susceptible to degradation by oxygen and water molecules: Organic materials
are very sensitive to oxygen and water molecules which can degrade the device very fast.
So the main current disadvantage of an OLED is the short lifetime."
•Low glass transition temperature Tg for small molecular devices (>70 degree C). So the
operating temperature cannot exceed the glass transition temperature."
•Low mobility of holes and electrons due to amorphous nature of the organic molecules."
•Low stability at high brightness levels."
•Low device efficiency."
•Device complexity: may affect the cost of manufacturing."
•Difficulty in fabricating uniform, large-area lighting sources."
•Non-existent infrastructure.
Disadvantages
Different Types of OLEDs
Passive-matrix OLED (PMOLED)
PMOLEDs have strips of cathode, organic layers and strips of anode. The anode strips are
arranged perpendicular to the cathode strips. The intersections of the cathode and anode
make up the!pixels!where light is emitted. External circuitry applies current to selected
strips of anode and cathode, determining which pixels get turned on and which pixels
remain off. Again, the brightness of each pixel is proportional to the amount of applied
current.
PMOLEDs are easy to make, but they consume more power than other
types of OLED, mainly due to the power needed for the external circuitry.
PMOLEDs are most efficient for text and icons and are best suited for
small screens (2- to 3-inch diagonal) such as those you find in!cell
phones,!PDAs!and!MP3 players. Even with the external circuitry, passive-
matrix OLEDs consume less battery power than the LCDs that currently
power these devices.
Active-matrix OLED (AMOLED)
AMOLEDs have full layers of cathode, organic molecules and anode, but the anode layer
overlays a thin film transistor (TFT) array that forms a matrix. The TFT array itself is the
circuitry that determines which pixels get turned on to form an image.
AMOLEDs consume less power than PMOLEDs because the TFT array
requires less power than external circuitry, so they are efficient for large
displays. AMOLEDs also have faster refresh rates suitable for video. The best
uses for AMOLEDs are computer monitors, large-screen TVs and electronic
signs or billboards.
OLED Deposition Techniques
Deposition techniques involving OLED materials can be classified as wet or dry techniques.
Dry techniques such as vacuum thermal evaporation (VTE) and organic vapor phase
deposition (OVPD) have been predominantly used for small molecular organic material
deposition. This is because of the ease of depositing large area uniform and homogenous
film using these dry techniques, and because the solubility of small aromatic molecules
tends to be too small for solution processing of sufficiently thick films."
Polymer organic materials are deposited using wet techniques such as spin coating, ink jet
printing, and contact stamping. Typical dry techniques such as vacuum thermal evaporation
are not viable due to their large molecular weight which causes their evaporation
temperature to be far in excess of their decomposition temperature.
Spin coating is a fast and easy deposition technique as depicted in Fig. 3.3, in which a
quantity of solid (usually polymer) is dissolved into an organic solvent. This solution is placed
onto a substrate, allowed to wet the entire area to be coated, and then the spinning speed is
typically set at 1000-10000 rpm.
Spin Coating
The centrifugal force of spinning causes majority of the solution to expel; however a fraction
of the solution is left behind. The exact thickness of this liquid film is difficult to predict, but is
controlled by a combination of adhesion forces at the substrate/liquid interface, solution
viscosity, and friction at the air/liquid interface. For a low pressure solvent, this thin liquid film
can exist indefinitely on the spinning substrate surface. However, for typical organic solvents
used in spin coating, the vapor pressure is quite high, and the solvent begins to evaporate
immediately upon exposure to any unsaturated environment. Thus, this thin liquid film
eventually decreases in thickness (time scales are typically 1-60s), leaving behind an even
thinner (1-1000nm), flat solid film of the initial solvated liquid. It is important to note that with
the spin coating deposition technique, patterned deposition is not possible.
Vacuum Thermal Evaporation
VTE is the simplest deposition technique. A boat made out of a resistive metal (typically tungsten,
molybdenum or tantalum) is heated by passage of electric current. The boat contains organic
material, which upon heating evaporates or sublimes. The evaporation takes place in a low
pressure (typically 10-6 torr) vacuum chamber so that the evaporated material is unlikely to
undergo any collisions along its path towards the substrate, and also to keep the deposited
materials as pure as possible.
Each molecule that is liberated from the solid has some initial speed and direction that is not
changed until it contacts a substrate or chamber surface, which is typically cold enough to
cause it to condense on contact. Because of the even distributions of the initial molecular
trajectories, the resulting deposition is essentially of uniform thickness for any solid angle
intersected by a substrate. Using this technique, 70-99% (depending on throw distance) of the
material in the boat is deposited on the walls of the vacuum chamber rather than the substrate
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